In electrophotographic imaging processes, such as in electrophotographic copying machines employing reusable photoconductive elements an electrostatic latent-image charge pattern is formed on the photoconductive element which consists of one or more photoconductive layers deposited on a conductive support and can be in the form a belt, drum or plate. By treating the charge pattern with a dry developer containing charged toner particles, the latent image is developed. The toner pattern is then transferred to a receiver such as a sheet of paper to which it is fixed by fusion or other means.
In the most effective modern photocopiers, the photoconductive layers of the element contain organic photoconductors dispersed in a binder resin matrix. To permit long, continuous use of these reusable elements, the binder resin must be tough and strong. A problem, however, in transferring the developed image to a receiver is that the attraction of the toner to the surface layer of the electrophotographic elements which employ the usual kinds of tough organic binder resins can cause incomplete transfer of toner. The resulting transferred image on the receiver has hollow characters and other defects. The problem is especially severe when the image is transferred by pressing a receiver element such as a paper sheet into contact with the toned surface of the photoconductive element.
Efforts to solve the image-transfer problem have included providing abhesive or release coatings to the surface layers of photoconductive elements. A drawback of this attempt to solve the problem is that an insulating, non-photoconductive overcoat can interfere with the photoconductive properties of the element. If the coating is thick, it can materially reduce the electrophotographic speed or sensitivity. Even if thin, an insulating overcoat layer can shorten the life of a photoconductor to such an extent that it cannot be regenerated for repeated use. This is believed to be caused by the trapping of residual charges between the insulating coating and the photoconductive layer. If the surface is merely coated with a soft release substance such as a metal stearate, the coating rapidly wears off and the transfer problem reappears. There is a need, therefore, for a binder composition for the surface layer of photoconductive elements which provides suitable surface properties for good image transfer without the necessity for release overcoats and yet which also has the physical strength required of binders in reusable photoconductive elements and moving belts. Additionally, it is advantageous for such a binder resin to be compatible with the formation of high-speed aggregate photoconductive compositions such as those disclosed, e.g., in the U.S. Pat. Nos. to Light, 3,615,414; Contois, 4,350,751; Berwick et al, 4,175,960; Stephens et al, 3,679,407; Gramza eta al, 3,684,502; and Contois et al, 3,873,311.
In accordance with the present invention, a novel electrophotographic element is provided which is strong enough for repeated use but from which, even after many cycles for use, toner can be transferred to a receiver element by pressure rollers or other means with minimal image defects. The novel element also permits the use of aggregate photoconductive compositions in its surface layer.